1. Field of the Invention
The present invention relates generally to fluid loss control methods, and more particularly, to methods of temporarily stopping fluid losses from well bores into subterranean formations by way of perforations extending therebetween.
2. Description of the Prior Art
In the completion of oil and gas wells, high density fluids must often be used to prevent blowouts during the performance of procedures such as perforating, gravel packing and the like. High density completion fluids are weighted with density increasing materials such as calcium chloride, calcium bromide, zinc bromide, barium sulfate and the like whereby the hydrostatic head of the completion fluid within a well bore and exerted on subterranean formations penetrated thereby is greater than the fluid pressures within the formations. This positive pressure towards the subterranean formations causes the completion fluid to be lost into the formations unless one or more fluid loss control agents are included in the completion fluid.
The loss of high density completion fluids is very undesirable in that the completion fluids are very expensive, and if high losses are experienced thousands of dollars in cost can be lost. Another problem is that high density completion fluids must be relatively viscous in order to suspend weighting materials included therein. When the viscous fluids are lost into formations containing hydrocarbons, the permeabilities of the formations can be reduced which in turn reduces the ability to produce hydrocarbons from the formations. In addition, some formations containing hydrocarbons also contain clays which swell when contacted by the highly saline completion fluids. The clays then migrate with fluids produced from the formation causing further formation permeability damage, etc. Thus, it is mandatory that the loss of high density completion fluids to subterranean formations be prevented.
A variety of additives have heretofore been included in completion fluids to reduce fluid loss. Most of such additives fall into the categories of particulates or gels. Particulate fluid loss materials function as fluid loss control agents by bridging on the faces of permeable formations. They are sized to form low permeability barriers which restrict completion fluid flow into the formations. The heretofore used particulates have been of both the soluble and insoluble types. Soluble particulates are removed by washing or flushing the well bore with one or more solvents which dissolve the soluble particulates, or the soluble particulates are dissolved in hydrocarbon fluids subsequently produced from one or more formations penetrated by the well bore. Examples of particulate fluid loss additives which are soluble in subsequently produced hydrocarbons are disclosed in U.S. Pat. No. 3,319,716 issued on May 16, 1967. Insoluble particulates such as sized sand have also been utilized which are subsequently removed by hydrocarbon production.
Gel fluid loss additives, known as gel "pills" are made from various natural polymers such as guar gum, cellulose and their derivatives. The polymers are added to the completion fluid in relatively high concentrations, or lower concentrations are utilized and the polymers are crosslinked. A particularly successful fluid loss reducing gel is hydroxyethylcellulose. Simple non-crosslinked hydroxyethyl-cellulose gels do not bridge on the formation, but rather enter the formation adjacent the well bore whereby a high viscosity gel barrier reduces fluid loss. The gel is subsequently broken by internal or external gel breaking chemicals so that hydrocarbon fluids can be produced from the formation. However, non-crosslinked hydroxyethylcellulose gel only slows the rate of fluid loss, and generally, the fluid loss can not be completely stopped without including a particulate material which bridges on the face of the formation.
In the use of high density completion fluids, soluble particulates that bridge on the formation face and essentially completely stop fluid loss have heretofore been included in the completion fluids. However, when such completion fluids are used in well bores which include casing or liners cemented therein and the completion fluids must flow through perforations formed through the cemented casing or liners in order to bridge on the face of the formation, impenetrable plugs can be formed in the perforations which can not subsequently be removed. For example, when the soluble solid particulates bridge on a formation face and fill the perforations extending from within the well bore into the formation, solvents for the soluble particulates subsequently circulated through the well bore have limited contact with the particulates. The solvents can not penetrate the perforations and do not remove the particulates from the perforations. In a number of completion procedures such as gravel packing completions, it is not possible to flow the producing formation to remove the particulates prior to placing the gravel pack, and the particulates can be locked in place by the gravel pack sand. This can stop or drastically reduce hydrocarbon production from the formation requiring the performance of costly reperforation and other remedial procedures.
Thus, there is a need for an improved method of preventing high density completion fluid loss using soluble particulate materials whereby the soluble particulate materials can be readily removed from perforations extending from the well bore into the formation.